1. Field of the Invention
The present invention relates to a fuel cell, a fuel cell stack, and a method for manufacturing a fuel cell.
2. Description of the Related Art
A fuel cell stack has a structure formed by stacking a number of fuel cells. A stack of the number of fuel cells is fastened by end plates arranged on both sides in the stacking direction.
The fuel cell includes a membrane electrode assembly (hereinafter abbreviated as “MEA”). The MEA includes an electrolyte membrane and a pair of electrodes. The pair of electrodes are arranged on both sides of the electrolyte membrane in the thickness direction. One of the pair of electrodes is a fuel electrode, and the other is an oxygen electrode. The fuel electrode is supplied with fuel gas such as hydrogen and hydrocarbon. The oxygen electrode is supplied with oxidant gas such as oxygen and air. The supplied gas, the electrolyte membrane, and the electrodes are subjected to an electrochemical reaction at three phase boundary to generate electricity.
For example, Japanese Patent Application Publication No. JP-A-2002-260693 discloses a fuel cell having a seal structure for suppressing mixture (crossover) of the fuel gas and the oxidant gas. FIG. 12 shows a partial sectional view of the fuel cell disclosed in the document. An outer periphery portion of the fuel cell is shown in FIG. 12. As shown in FIG. 12, a fuel cell 100 includes an MEA 101 and a gasket 102. The MEA 101 includes an electrolyte membrane 101a and a pair of electrodes 101b and 101c. The pair of electrodes 101b and 101c are arranged on both surfaces in the thickness direction of the electrolyte membrane 101a. The gasket 102 has a three-layer structure formed by stacking an elastic layer 102a, a shape preservation layer 102b, and a thermal cross-linking layer 102c. 
The thermal cross-linking layer 102c bends and extends in an L-shape. Specifically, the thermal cross-linking layer 102c extends from a thickness direction gap C100 between the electrolyte membrane 101a and the shape preservation layer 102b to a surface extending direction gap C101 between the elastic layer 102a as well as the shape preservation layer 102b and the electrode 101b. In a thickness direction end portion of the thermal cross-linking layer 102c, a rib 102d is formed.
A method for manufacturing the fuel cell 100 includes a gasket original sheet preparation step, a gasket original sheet punching step, an in-cavity arrangement step, and a cross-linking step. FIG. 13 shows a schematic view of the gasket original sheet preparation step. FIG. 14 shows a schematic view of the gasket original sheet punching step. FIG. 15 shows a schematic view of the in-cavity arrangement step.
In the gasket original sheet preparation step, as shown in FIG. 13, an original sheet of the gasket 102 having the three-layer structure is prepared. The original sheet of the gasket 102 is formed by stacking the elastic layer 102a, the shape preservation layer 102b, and the uncross-linked thermal cross-linking layer 102c. In the gasket original sheet punching step, as shown in FIG. 14, a hole such as a manifold hole 103 is punched out in a predetermined portion of the original sheet of the gasket 102.
In the in-cavity arrangement step, as shown in FIG. 15, the MEA 101 and the original sheet of the gasket 102 are stacked and arranged in a cavity of a metal mold 104. The thermal cross-linking layer 102c is arranged only in the thickness direction gap C100. Next, a mold clamping of the metal mold 104 is performed. The thermal cross-linking layer 102c is compressed in the thickness direction by the mold clamping. Therefore, the thermal cross-linking layer 102c flows to spill out in the surface extending direction, and is filled in the surface extending direction gap C101 as shown with white arrows in FIG. 15. In the cross-linking step, the thermal cross-linking layer 102c is cross-linked. Thus, the fuel cell 100 is manufactured.
With the fuel cell 100 disclosed in the document, the thermal cross-linking layer 102c and the electrolyte membrane 101a are subjected to cross-linking adhesion. Therefore, the thermal cross-linking layer 102c and the electrolyte membrane 101a are firmly joined. Thus, a crossover due to separation of the electrolyte membrane 101a and the gasket 102 can be suppressed.
However, in the fuel cell 100 disclosed in the document, an end surface F100 of the electrolyte membrane 101a is exposed to the manifold hole 103, as shown in FIG. 12. Therefore, there has been a possibility of a product due to decomposition of the electrolyte membrane 101a, such as a fluoride and a sulfonic acid, leaking from the end surface F100.
In this regard, Japanese Patent Application Publication No. JP-A-9-199145 discloses a fuel cell in which an outer periphery portion of an electrolyte membrane is sealed with an epoxy adhesive. In the fuel cell disclosed in the document, an end surface of the electrolyte membrane is covered by the adhesive. Therefore, the possibility of a product leaking from the end surface of the electrolyte membrane is small.
However, with the fuel cell of the Japanese Patent Application Publication No. JP-A-9-199145, it is necessary to form an ion exchange section in the outer periphery portion of the electrolyte membrane in order to improve the adhesiveness of the electrolyte membrane and the adhesive. In order to form the ion exchange section, it is necessary to mount a predetermined masking tool to the electrolyte membrane, and then immerse the electrolyte membrane in a potassium hydroxide solution. This operation is complicated. Also, the necessity for this operation leads to an increase in manufacturing cost of the fuel cell and consequently the fuel cell stack.
Japanese Patent Application Publication No. JP-A-2007-157420 discloses a fuel cell in which an outer periphery portion of an electrolyte membrane is sealed with an annular ribbon having a thermoplastic resin layer. In the fuel cell disclosed in the document, an end surface of the electrolyte membrane is covered by the annular ribbon. Therefore, the possibility of a product leaking from the end surface of the electrolyte membrane is small.
However, in the fuel cell of the Japanese Patent Application Publication No. JP-A-2007-157420, the annular ribbon is sandwiched merely by a pair of gaskets from both sides in the thickness direction. Therefore, the annular ribbon tends to fall from between the pair of gaskets. Also, there is a possibility of the thermoplastic resin layer of the annular ribbon deforming due to heat. Therefore, the thermoplastic resin layer may separate from the end surface of the electrolyte membrane due to heat. Thus, the sealability at the end surface of the electrolyte membrane may deteriorate.